The working principle of a linear electrodynamic engine is based on the generation, by induction coils, of cyclic magnetic forces that stimulate with a straight movement at least one movable mass, or piston, mounted on a mechanical bearing, which due to its construction develops an axial elastic return force proportional to the displacement of the piston. The latter is therefore characterized by a mechanical resonant frequency determined by the movable mass, the stiffness of the bearing, the magnetic stiffness and the fluid load.
The control of the engine then consists in applying to the induction coils an excitation current at the mechanical resonant frequency of the piston and its bearing, so as to obtain a natural amplification of the displacement movement of the piston.
However, the efficiency of such a mechanism depends largely on the damping forces, in particular friction forces, that tend to oppose the movement of the piston proportionally to its speed. In fact, the optimum performance is obtained when the natural amplification at the mechanical resonance is at a maximum, i.e. when the damping forces of the piston are at a minimum. For this reason, great efforts have been made during the design of this type of engine in order to minimize the causes of damping at the piston.
As a result, therefore, such engines have very low damping properties, which makes them particularly sensitive to external mechanical stresses, especially vibrations and impacts. This is because every vibration or impact applied to the interfaces of the engine close to the mechanical resonant frequency is amplified by the piston. This converts into large amplitude oscillations of the piston, greater than the physical limits imposed by the size of the compressor. Internal impacts may then be produced between the piston and the mechanical stops of the compressor, thus producing different types of damage capable of impairing the performance of the engine in terms of efficiency and lifetime.
It will be understood that these drawbacks are particularly disadvantageous in the case of engines that are to be loaded on spacecraft. The severe mechanical environment during the launch of the craft indeed cause vibrations and repeated impacts of the pistons of the engine against the compressor case. In addition, taking account of the lifetimes demanded, of 5 to 10 years, it is not only necessary to pass through the launch step without damage, but also to ensure optimum operation throughout the complete duration of the mission. Yet, the impacts undergone throughout the duration of the launch do not make it possible to guarantee these objectives and constitute a potential cause of mission failure.
Various solutions have already been proposed for limiting the displacement of the pistons of linear electrodynamic engines when they are stressed by external forces such as impacts and vibrations.
A first solution consists in short-circuiting the induction coils of the engine. When the piston is set in motion by external mechanical loads, the magnetic flux in the coils varies and, if the coils are short-circuited, a counter-electromotive force appears in the coils according to Lenz's Law and generates a magnetic force that opposes the displacement of the piston, which creates damping.
However, in this known solution the magnetic force generated remains low and insufficient to damp high vibration levels, all the more so as it is defined by the fixed physical dimensions of the coils and of the magnets and is therefore not adjustable.
A second solution proposes employing position sensors in the compressor in order to know the position and the speed of the pistons at every moment. Control electronics interpret this information and provide the electrical energy necessary to keep the pistons in their neutral position.
The drawbacks of this solution are an employment of position sensors that is difficult, expensive and risky in terms of reliability and lifetime, along with control electronics that are complex and also expensive.
A third solution consists in introducing a mechanism for blocking the piston into the compressor.
This third solution is very difficult to implement in practice and risky in terms of reliability.